Thermally expandable microcapsules are used as design-imparting agents and weight-reducing agents in various applications. They have also been used for paint, such as foaming ink and wallpaper, to achieve weight reduction.
A widely known thermally expandable microcapsule is one which includes a thermoplastic shell polymer filled with a volatile expansion agent which turns into gas at a temperature not higher than the softening point of the shell polymer. For example, Patent Literature 1 discloses a method for producing a thermally expandable microcapsule filled with a volatile expansion agent, including the steps of: preparing an oily mixture by mixing a monomer and a volatile expansion agent such as an aliphatic hydrocarbon having a low boiling point; and adding the oily mixture and an oil-soluble polymerization catalyst to an aqueous dispersion medium containing a dispersant with stirring to perform suspension polymerization.
The thermally expandable microcapsule obtained by the method can be thermally expanded by gasification of the volatile expansion agent at relatively low temperatures of about 80° C. to 130° C. When the microcapsule is heated at high temperatures or for a long time, however, gas escapes from the expanded microcapsule, leading to reduction in the expansion ratio. Also, insufficient thermal resistance and strength of the thermally expandable microcapsule may cause a phenomenon called “deflation” to break the microcapsule at high temperatures.
Patent Literature 2 discloses a thermally expandable microcapsule which includes as a shell a polymer obtainable by polymerizing a carboxy group-containing monomer and a monomer having a group reactive with carboxy groups. Patent Literature 2 teaches that such a thermally expandable microcapsule has an increased three-dimensional cross-linking density, and is therefore extremely resistant to contraction even after the shell is expanded to be very thin. Patent Literature 2 also teaches that such a thermally expandable microcapsule has significantly improved heat resistance.
However, formation of strong three-dimensional cross-links during polymerization inhibits expansion in foaming, and thus the resulting expansion ratio is yet insufficient.